1. Field of the Invention
This invention relates to electrophotographic photosensitive members and more particularly to an electrophotographic photosensitive member comprising an organic photoconductive material which can provide improved electrophotographic characteristics to the member.
2. Description of the Prior Art
Inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, etc. have so far been known as those for use in electrophotographic photosensitive members. While having many advantages, for example, chargeability in the dark to a suitable potential, little dissipation of charge in the dark, and fast dissipation possible on light irradiation, these photoconductive materials have various disadvantages. For example, a drawback of selenium type photosensitive members is that the crystallization of photoconductive materials readily proceeds by the action of factors such as temperature, humidity, dust, pressure, and the like, which becomes remarkable when the atmospheric temperature exceeds 40.degree. C., resulting in decreased chargeability and white spots on the developed images. Cadmium sulfide type photosensitive members have a drawback in that they cannot maintain stable photosensitivity under high humidity environmental conditions. While zinc oxide type photosensitive members need sensitization with a sensitizing colorant, a typical example of which is Rose Bengal, a drawback of these photosensitive members is that stable formations of images for a long period of time are impossible because such a colorant promotes the deterioration of chargeability by corona charging and undergoes fading upon light exposure.
On the other hand, various organic photoconductive polymers have been proposed including polyvinylcarbazole. Although superior to the above inorganic photoconductive materials in film-forming property and lightness, these polymers have been rarely put to practical use until now, since the film-forming property thereof is not yet sufficient and they are inferior to inorganic photoconductors in sensitivity, durability, and stability to changes of environmental conditions. Low molecular weight organic photoconductive materials also have been proposed, including hydrazone compounds as disclosed in U.S. Pat. No. 4,150,987, triarylpyrazoline compounds as disclosed in U.S. Pat. No. 3,837,851, and 9-styrylanthracene derivatives as disclosed in Japanese Pat. Kokai Nos. 94828/1976 and 94829/1976. These organic photoconductive materials have overcome the drawback of insufficient film-forming property, which has been a problem in the field of organic photoconductive polymers, by incorporating therein a suitable binder. However, they lack sufficient sensitivity.
In view of the above, photosensitive members of laminate structure have been recently proposed wherein the photosensitive layer has been divided into two layers functioning separately as a charge generation layer and a charge transport layer. The electrophotographic photosensitive member having such photosensitive layers of laminate structure is improved in sensitivity to visible light, electric charge retention, and surface strength. These photosensitive members have been disclosed in U.S. Pat. Nos. 3,837,851 and 3,871,882.
Nevertheless, electrophotographic photosensitive members employing conventional organic photoconductive materials have deficiencies to be corrected, since their sensitivity is still insufficient and variations in light portion potential and dark portion potential are significant when they are charged and exposed repeatedly.